There is a problem with finding useful and free APIs for TA indicators.
Different APIs:
Alpha Vantage (rapidapi, 1 req/15sec, 500 requests/day) - https://rapidapi.com/alphavantage/api/alpha-vantage/pricing
Taapi (best indicators) - https://taapi.io/indicators/relative-strength-index-rsi/
Very best results, 200 requests/day - https://technical-analysis-api.com/documentation#tag/Strategy
я не понимаю как работает RSI на этом сайте
График Trading view - https://www.tradingview.com/HTML5-stock-forex-bitcoin-charting-library/
Creating google extension - https://ankitkumar.dev/building-own-cheome-extension/